PROJECT SUMMARY Bipolar disorder (BPD) is an affective disorder causing life-long cycling between mania and depression, afflicting 1-2% of the population and resulting in suicide in 15% of patients. The etiology of BPD is unknown, and first-line treatments such as lithium have no known mechanism of action. Our lab has shown that lithium directly inhibits GSK-3, that lithium activates Wnt/?-catenin?a signaling pathway inhibited by Gsk3?in the brain, and that GSK-3 inhibition mediates the behavioral effects of lithium in mice. This proposal combines behavioral analysis with cutting-edge brain imaging techniques and modern mouse genetics to define with cellular resolution the cells that are directly targeted by lithium in the brain, track cell fate decisions following lithium-induced neurogenesis, and test the requirement for Gsk3, using conditional genetic deletion, in mediating the behavioral and neurogenic effects of lithium. The first specific aim takes an unbiased approach to identify which neurons and other cell types in the brain directly respond to lithium using a novel fluorescent reporter for Wnt/?-catenin activity as a surrogate for GSK-3 inhibition. We will further identify the cell fate decisions of adult hippocampal neural stem cells (NSCs) undergoing lithium-induced neurogenesis to determine whether the pro-neurogenic effect of lithium is likely to play a direct role in its therapeutic mechanism of action. The second specific aim asks whether Gsk3 loss specifically in postnatal forebrain neurons is required to mimic lithium's effects on behavior, which was not tested in previous studies of GSK-3 inhibition on behavior. Furthermore, we will determine whether deletion of Gsk3 in specifically adult NSCs phenocopies lithium's effects on adult hippocampal neurogenesis and NSC fate decisions. The research strategy described in this proposal will provide the applicant, Dr. Melinda Snitow, comprehensive training in the field of neurogenesis, mouse neurobehavioral testing, and state-of-the-art imaging techniques, which are critical to develop a career studying the etiology and treatment of BPD. The proposed site of research, The University of Pennsylvania's Perelman School of Medicine, provides comprehensive state-of-the-art physical resources, combined with an extensive community of experts in neuroscience and behavior. The sponsor, Dr. Peter S. Klein, is a leading expert on the molecular and behavioral effects of lithium and Gsk3. Dr. Klein's extensive experience in the field of BPD research and pharmacology, and collaborations with neurogenesis and behavior experts Dr. Amelia J. Eisch, Dr. Hongjun Song, and Dr. Elizabeth A. Heller, plus consultation regarding clinical relevance with Dr. Chang-Gyu Hahn, will provide ideal training in behavioral neuroscience and neuropsychopharmacology to prepare Dr. Snitow for a career in BPD research as an independent PI. The proposed research will elucidate lithium's direct cellular targets that regulate behavior and neurogenesis, and lay the foundation for future studies of lithium's molecular mechanisms in the appropriate cellular context.